MCC is a purified partially de-polymerized crystalline polymer that has many industrial uses. The known commercial processes for making MCC use partial acid hydrolysis of purified cellulose under conditions, at which only the amorphous areas of the polysaccharides are hydrolyzed, dissolved and removed. The crystalline cellulose areas are not hydrolyzed and can be recovered. The acid hydrolysis process is generally considered completed when a level off degree of polymerization (“LODP”) cellulose product is obtained. As disclosed in U.S. Pat. No. 2,978,446, starting with purified wood celluloses, such an acid hydrolysis produces MCC with the LODP in the range of 50 to 200. As described in O. A. Battista, P. A. Smith, “Microcrystalline Cellulose,” Industrial and Engineering Chemistry, vol. 54, no. 9, p. 24 (September 1962), high alpha cellulose has an average degree of polymerization (DP) of more than 1000 and microcrystalline cellulose has an average DP of about 140 to 190. Also described in S. Rydholm, “Pulping Process,” Textbook of John Wiley & Sons, Inc., pp. 106–07 (1965), the average DP of isolated and purified cellulose ranges from 1000 to as high as 5000 depending on the particular wood species and isolation method. Commercial MCC has been specified in US Pharmacopeia (USP 23 NF 18) to contain not less than 97% cellulose.
In the known conventional MCC processes, purified celluloses, such as purified pulps, are used for preparing MCC. These purified pulps are prepared from wood by prehydrolysis of wood chips under acidic conditions, alkali pulping of the prehydrolyzed wood chips and purification of the resultant pulp. As described in Simmons et al., Tappi, vol. 39, no. 9, pp. 641–47 (1956) and Tappi, vol. 38, no. 3, pp. 178–85 (1955), purified pulp is high in alpha cellulose content, in excess of 97%, and contain low levels of hemicellulose or pentosan impurities, less than 2%. Such purified pulps are also commonly known as dissolving pulps and the described method is still being practiced in the industry. Mention is also made of U.S. Pat. No. 5,589,033, which discloses a process to produce a higher quality dissolving pulp in which the hydrolyzate liquor from the prehydrolysis step is removed from the wood chip-cooking vessel, prior to alkali pulping with sodium sulfide and sodium hydroxide.
Dissolving pulps are relatively expensive to produce and their use greatly increases production costs of MCC. Dissolving pulps may be produced from kraft, soda or sulfite pulp by bleaching and other treatments. Dissolving pulps are used as a starting material for a number of products such as viscous rayon, cellulose esters, cellulose ethers, such as taught in Hyatt et al. U.S. Pat. No. 6,057,438. They are also used to make cigarette tow.
When wood chips are not prehydrolyzed before alkali pulping and purification, as described in Richter, Tappi, vol. 38, no. 3, p. 147 (1955) their alpha cellulose content is less than 90% and pentosan content is as high as 10% for softwoods and 20% for hardwoods. Also from Richter, Table XVI and FIG. 15, a cold caustic treatment of unbleached softwood kraft pulp could only reduce its pentosan content from 8.6% to 3.2% and any increase in the caustic concentration of the solution beyond 10–12% resulted in pulps with a higher residual pentosan content.
The basic method for preparing MCC from purified pulps was first described in Battista et al., U.S. Pat. No. 2,978,446, which still represents the basis for many conventional MCC manufacturing processes. In Battista et al. '446 the initial step in the process is the repulping of dry dissolving pulp. The repulped material is then acid hydrolyzed with a mineral acid, such as HCl or H2SO4 to dissolve the amorphous cellulose. The material is then dried, milled and bagged. This process is generally performed in a batch-type method.
There are a number of disadvantages with the Battista et al. process and other conventional MCC processes. The starting material is required to be a purified cellulose material that is high in alpha cellulose content. For example, the raw material for a commercially available MCC, Avicel®, is stated to be a special grade of alpha purified wood cellulose. Industrial and Engineering Chemistry, vol. 54, no. 5, pp. 20–28. Thus, it would represent a notable advance in the state of the art if the MCC could be prepared from a pulp that was not required to undergo the expensive purification processes of the prior art, such as directly from a paper-grade pulp.
Attempts in the prior art to employ other than purified celluloses have not been well received due to their inherent deficiencies and poor economics. For example, to produce MCC from partially purified cellulose, with an alpha cellulose content of 92.2%, U.S. Pat. No. 5,543,511 discloses a method for producing MCC using pressurized oxygen and/or carbon dioxide and high temperature conditions. From unpurified cellulosic material, U.S. Pat. No. 5,769,934 describes a steam explosion technique to remove lignin and hemicellulose prior to MCC manufacturing.
For preparing MCC from materials containing lignin, hemicellulose and cellulose, U.S. Pat No. 6,228,213 discloses a combination of reactive extrusion in the presence of basic solution followed by reactive extrusion in the presence of acid. The extrusion in the first step, in the presence of sodium hydroxide, is carried out at temperatures ranging from 140° C. to 170° C. The extrusion in the second step, in the presence of an acid, is carried out at a temperature of 140° C. The final extruded product is bleached with hydrogen peroxide or hypochloride prior to being spray dried into MCC powder.
Additionally, acid depolymerization of cellulosic material is known as an essential step in obtaining MCC in order to remove the amorphous cellulose material. One of the other problems with the prior art processes is that when performed on a commercial scale, the acid depolymerization step used in MCC manufacturing requires large quantities of acid. Sulfuric acid is generally used at 50% concentration in order to depolymerize cellulose pulps. Consequently, a large amount of alkaline agent has also been required to neutralize and wash the hydrolyzate after the acid treatment step. Thus, it would represent a significant advance in the state of the art if a process for producing MCC could be developed where the acid and or alkali agents are readily available and could readily be recycled for reuse.
In attempts to move away from acid treatment steps, the prior art has also explored the possibility of using enzymes and/or microorganisms to produce MCC. For example, the previously mentioned Hyatt et al. '438 patent teaches a process for preparing dissolving grade pulps by a process sequence of caustic extraction, xylanase treatment and caustic extraction to remove xylan from the paper grade pulp. The process increases the high alpha cellulose content of the wood pulp from less than 85% to more than 97% and decreases its hemicellulose impurity from more than 15% to less than 3%. In the two extraction stages, the patent teaches that sodium hydroxide concentrations of not more than 8–12% and temperatures of not less than 50–100° C. need to be employed in order to prevent an undesirable transformation of cellulose I into cellulose II structure.
Zabriskie, U.S. Pat. No. 4,427,778 teaches a process for converting cellulose to MCC using a cellulase enzyme. An example of a use of microorganisms for converting cellulose is found in Kawai et al. U.S. Pat. No. 4,943,532. These methods of using enzymes or microorganisms are expensive and require the addition of another reagent not readily available at a paper plant. Thus, there is a need for a process for producing MCC that can employ paper-grade pulp as the starting material and be readily integrated with an existing paper making-plant.
Mention is also made of U.S. Pat. No. 5,574,150 that discloses a process to produce MCC powder with a good balance between compactibility property and rate of disintegration property. The patent also discloses that products with a low apparent specific volume after tapping were said to be more desirable for the tablet manufacturing in the pharmaceutical industry.